It’s yet another of those annoying glitches in spelling you have no choice but to learn by rote. We also use the u-free spelling for ‘honorarium’ and ‘honorific’, but I suspect ‘honorary’ is the word you’ll encounter most often.

This is one case where US English spelling is more consistent: since Americans spell ‘honour’ without a ‘u’ as ‘honor’ in all circumstances, there’s no exception to learn here. However, that doesn’t mean you can apply the principle in reverse and start using ‘honourary’ in Australia. Indeed, it seems more likely that in the future, we’ll see the US spelling become more common in other contexts — but for plain old ‘honour’ as a noun, we haven’t crossed that barrier yet, so you need to know when to use a ‘u’ and when to leave it out.

Ultimately, English spelling is not consistent in any version, and everyone simply has to learn the exceptions. Accuracy matters.

The -or endings are a letter shorter than the -our ones, but still irregular. The main pattern is -er, in several hundred words (e.g. brother, potter, winner, fibber, grabber). Their -ize endings merely undermine the main pattern for a final /z/ sound which is -se (rise, rose, wise). They were adopted to make US spelling different, not better.

Their best change was to reduce the 'to practise - a practice' differentiation to just 'practice'. All the 333 heterographs for identical words like 'there/their', 'it's/it's', 'to/too/two' do nothing but waste school time. Over 2000 English words with several meanings have just one spelling (mean, lean, sound) and cause no difficulties whatsoever. The heterographs merely ensure that learning to write English takes far longer than need be and not too many people become confident writers.